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Concepts of CDMA Introduction

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1 Concepts of CDMA Introduction
Objectives: This presentation gives an overview of the proposed CDMA cellular system defined by the TIA 45.5 sub-committee. This system is largely based on the CDMA system developed by Qualcomm. The intent of this paper is to provide insight into the technology of CDMA and to describe some of the operating features of the proposed TIA CDMA system.

2 Cellular Access Methods
Power Frequency Time FDMA TDMA CDMA The Problem of Access: The personal communication industry is faced with the problem of an ever increasing number of users sharing the same limited frequency bands. To expand the user base, the industry must find methods to increase capacity without degrading the quality of service. The current analog cellular system uses a complex system of channelization with 30 kHz channels, commonly called FDMA (Frequency Division Multiple Access). To maximize capacity, FDMA cellular uses directive antennas (cell sectoring) and complex frequency reuse planning. To further increase system capacity, a digital access method is being implemented called TDMA (Time Division Multiple Access). This system uses the same frequency channelization and reuse as FDMA analog and adds a time sharing element. Each channel is shared in time by three users to effectively triple system capacity. CDMA stands for Code Division Multiple Access and uses correlative codes to distinguish one user from another. Frequency divisions are still used, but in a much larger bandwidth (1.25 MHz). In CDMA, a single user's channel consists of a specific frequency combined with a unique code. CDMA also uses sectored cells to increase capacity. One of the major differences in access is that any CDMA frequency can be used in all sectors of all cells.

3 CDMA Analogy The correlative codes allow each user to operate in the presence of substantial interference. An analogy to this is a crowded cocktail party. Many people are talking at the same time, but you are able to understand one person at a time. This is because your brain can sort out the voice characteristics and differentiate them from the other talkers. As the party grows larger, each person has to talk louder, and the size of the talk zone grows smaller. This would be more dramatic if each conversation were in a different language. CDMA is similar, but the recognition is based on the code. The interference is the sum of all other users on the same CDMA frequency, both from within and without the home cell and from delayed versions of these signals. It also includes the usual thermal noise and atmospheric disturbances. Delayed signals caused by multipath are separately received and combined in CDMA. This will be discussed in greater detail later in this presentation.

4 CDMA is Also Full Duplex
Amplitude US Cellular Channel 384 Reverse Link Forward Link 45 MHz AMPS Frequency MHz MHz Amplitude Traditional cellular system are known as full duplex systems since two channels are used at the same time. This allows completely independent transmission to and from the mobile at the same time. In the North American Cellular system, the forward and reverse link channels are separated by 45 MHz. The EIA/TIA-95-B system is also full duplex and uses 1.25 MHz wide channels for both the forward and reverse directions. For cellular applications in North America, EIA/TIA-95-B CDMA uses the same 45 MHz separation between forward and reverse links that AMPS uses. For PCS applications, J-STD-008 CDMA uses a separation between forward and reverse links of 80 MHz. Other systems throughout the world may use other spacings. Reverse Link Forward Link CDMA 45 MHz Frequency MHz MHz

5 Cellular Frequency Reuse Patterns
3 6 2 1 4 5 7 1 One of the major capacity gains with CDMA is because of its frequency reuse patterns. The normal reuse pattern for analog and TDMA systems employs only one seventh of the available frequencies in any given cell. This could really be called frequency non-reuse. With CDMA, the same frequencies are used in all cells. If using sectored cells, the same frequencies can be used in all sectors of all cells. This is possible because CDMA is designed to decode the proper signal in the presence of high interference. FDMA Reuse CDMA Reuse

6 The CDMA Concept Interference Sources
Walsh Code Spreading Encoding & Interleaving Correlator Decode & Deinterleaving Baseband Data Background Noise External Interference Other Cell Interference Other User Noise 9.6 kbps 19.2 ksps kbps CDMA Transmitter CDMA Receiver 1.25 MHz BW f c 10 kHz BW -100 dB/Hz Spurious Signals CDMA starts with a narrowband signal, shown here at the full data rate of 9600 bps. This is spread with the use of specialized codes to a bandwidth of 1.23 MHz. When transmitted, a CDMA signal experiences high levels of interference, dominated by the coded signals of other CDMA users. This takes two forms, interference from other users in the same cell and interference from adjacent cells. The total interference also includes background noise and other spurious signals. When the signal is received, the correlator recovers the desired signal and rejects the interference. The is possible because the interference sources are uncorrelated to the desired signal.

7 CDMA Paradigm Shift Multiple users on one frequency
Channel is defined by code Capacity limit is soft For most people familiar with FM communication systems, a paradigm shift is needed to properly discuss CDMA. Here are some differences between CDMA and analog FM: Multiple users are on one frequency simultaneously A Channel is defined by the correlative code in addition to the frequency The capacity limit is soft. Capacity can be increased with some degradation of the error rate or voice quality.

8 CDMA Capacity Gains Processing Gain (Chan BW) (1) (1)
Capacity = _____________ X _____ X ____ X (Fr) (Data Rate) (S/N) (Vaf) (Chan BW) (1) (1) CDMA = ____________ X _____ X _____ X (0.67) (1,230, ) (1) (1) (9,600) (5.01) (.40) CDMA = 42 Calls ( Using 1.5 MHz BW ) To see how CDMA offers greater capacity, we need to look at its potential in a given bandwidth. Remember that for any cellular system, the capacity can be made arbitrarily large by adding more and more cells. A more realistic approach for a capacity comparison is the number of calls per used bandwidth. Installing CDMA in an existing AMPS analog cellular system requires that a minimum of 1.5 MHz of bandwidth be removed from analog service. While the actual spreading bandwidth of a single CDMA frequency is 1.23 MHz, a total of 1.5 MHz is required to provide guardbands to reduce potential interfere with adjacent analog channels. Additional CDMA frequencies added to the system will only require 1.23 MHz of bandwidth. In this configuration, a single CDMA cell will support 42 telephone calls. This is derived from the equation shown. The processing gain is the ratio of the CDMA final bandwidth divided by the encoded voice data rate. The signal-to-noise ratio required for good voice quality varies greatly with propagation conditions. On average, typical transmission conditions require a signal-to-noise ratio of about 7 dB to provide adequate voice quality. Translated into a ratio, the signal must be 5 times stronger than the noise. The parameter Vaf is the voice activity factor. Since CDMA uses a variable rate voice encoder, Vaf for CDMA is 0.4. Fr is the frequency reuse efficiency and Sg is the sectorization gain. For CDMA, Fr is 0.67 (in other words almost 70% reuse efficiency). The frequency reuse efficiency is not 1 since the additional interference produced by surrounding cells causes a reduction in capacity. If the CDMA cells use 3-way sectored antennas, Sg is about 2.6 (almost 3 times the capacity when using sectorization). Again, the sectorization gain is not 3 due to the increased interference from the surrounding sectored cells. Given the same amount of bandwidth, an AMPS system has a capacity per cell of only about 7 calls. This is because although AMPS would have 50 channels in 1.5 MHz of bandwidth, only one-seventh can be used in any given cell because of interference. Using sectors in analog does not improve the capacity per MHz since interference from adjacent sector still requires a complex frequency reuse plan. Sectorization in analog simply results in physically smaller cells. AMPS = 1.5 MHz / 30 kHz = 50 Channels Capacity = 50 Channels / 7 ( 1/7 Frequency Reuse ) AMPS = 7 Calls ( Using 1.5 MHz BW )

9 CDMA Makes use of Diversity
Spatial diversity Frequency diversity Time diversity Another aspect of CDMA is diversity. CDMA uses three types of diversity: Spatial diversity, Frequency diversity, and Time diversity.

10 CDMA Spatial Diversity
Multiple antennas at base station Multiple base stations for soft handoff Spatial diversity takes two forms: Two antennas: The base station uses two receive antennas for greater immunity to fading. This is the classical version of spatial diversity. Multiple base stations simultaneously talk to the mobile during soft handoff.

11 Spatial Diversity During Soft Handoff
MTSO Vocoder / Selector Base Station 2 Base Station 1 Land Link During Soft Hand-off, contact is made with two base stations simultaneously. The signals from the base to mobile are treated as multipath signals and are coherently combined at the mobile unit. At the base stations, the signals are transmitted via the network to the Mobile Telephone Switching Office (MTSO), where a quality decision is made on a frame-by-frame basis, every 20 ms.

12 CDMA Frequency Diversity
Combats fading, caused by multipath Fading acts like notch filter to a wide spectrum signal May notch only part of signal Amplitude Frequency 1.25 MHz BW Frequency diversity is inherent in spread spectrum systems. A fade of the signal is less likely than with narrow band systems. Fading is caused by multipath and is a function of the time delays in the alternate paths. In the frequency domain, a fade appears as a notch filter that moves across a band. As the user moves, the frequency of the notch changes. The width of the notch is on the order of one over the difference in arrival time of two signals. For a 1 µsec delay, the notch will be approximately 1 MHz wide. The TIA CDMA system uses a 1.25 MHz bandwidth, so only those multipaths of time less than 1 µsec actually cause the signal to experience a deep fade. In many environments, the multipath signals will arrive at the receiver after a much longer delay. This means that only a narrow portion of the signal is lost. In the display shown, the fade is 200 to 300 kHz wide. This results in a power loss to a CDMA signal, but could result in a the complete loss of an analog or TDMA signal.

13 CDMA Time Diversity Uses rake receiver Data is interleaved
Convolutional encoding Viterbi decoding Time diversity is a technique common to most digital transmission systems. However, the techniques used in CDMA are different from those used in GSM and TDMA. Signals are spread in time by use of interleaving. Forward error correction is applied, along with maximal likelihood detection. The particular scheme used for CDMA is convolutional encoding in the transmitter with Viterbi decoding using soft decision points in the receiver.

14 The Rake Receiver Amplitude Time Frequency
CDMA takes advantage of the multipath by using multiple receivers and assigning them to the strongest signals. The mobile receiver uses three receiving elements, and the base station uses four. This multiple correlator system is called a rake receiver. In addition to the separate correlators, searchers are also used to look for alternate multipaths and for neighboring base station signals. Frequency

15 CDMA Reverse Link Power Control
All mobiles are received at base station at equal power Two types of control: Open Loop Power Control Closed Loop Power Control Power Control: One of the fundamental enabling technologies of CDMA is power control. The power of all mobile units is controlled so as to arrive at the base station at an equal level. In this way, the interference from one unit to another is held to a minimum. Two forms of power control are used for the reverse link: open loop power control, and closed loop power control.

16 Open Loop Power Control
Assumes loss is similar on forward and reverse paths Receive Power + Transmit Power = -73 All powers in dBm Example: For a Received Power of -85 dBm (at the mobile) Transmit Power = (-73) - (-85) Transmit Power = +12 dBm Open loop power control is based on the similarity of the loss in the forward path to the loss in the reverse path (forward refers to the base-to-mobile link, while reverse refers to the mobile-to-base link). Open loop control sets the sum of transmit power and receive power to a constant, nominally -73, if both powers are in dBm. A reduction in signal level at the receive antenna will result in an increase in signal power from the transmitter. For example, assume the received power from the base station is -85 dBm. This is composite signal from the base station. The open loop transmit power setting would be +12 dBm.

17 Closed Loop Power Control
Directed by base station Updated every 1.25 ms 1 dBm step size Corrects Open Loop Power Estimate Closed loop power control is used to allow the power from the mobile unit to deviate from the nominal as set by open loop control. This is done with a form of delta modulator. The base station monitors the power received from each mobile station and commands the mobile to either raise power or lower power by a fixed step of 1 dB. This process is repeated 800 times per second, or every 1.25 ms. Because the power of the mobile is controlled to be no more than is needed to maintain the link at the base station, much less power is typically transmitted from the mobiles than is the case with analog. The analog radio needs to transmit enough power to maintain a link even in the presence of a fade. Most of the time it is transmitting with excess power. The CDMA radio is controlled in real time and is kept at low power. This has the benefit of longer battery life and smaller, lower cost amplifier design. If recent health concerns over cellular phone radiation are founded, CDMA will be preferred.

18 CDMA Variable Rate Speech Coder
20 Millisecond blocks of speech Full data rate of 9600 bps Lowest rate (1/8) of 1200 bps Mobile transmits in bursts of 1.25 msec Base repeats bits and lowers power System capacity increases by 1/Voice Activity Factor CDMA takes advantage of quiet times during speech to raise capacity. A variable rate vocoder is used; the channel is at 9600 bps when the user is talking. When the user pauses, or is listening, the data rate drops to only 1200 bps and 4800 bps are also used, though not as often as the other two. The data rate is based on speech activity and a decision as to the appropriate rate is made every 20 ms. Normal telephone speech has approximately a 40% activity factor. The mobile station lowers its data rate by turning off its transmitter when the vocoder is operating at less than 9600 bps. At 1200 bps, the duty cycle is only 1/8 that of the full data rate. The choice of time for this duty cycling is stochastic, so the power is lowered at all times when averaged over many users. Lowering the transmit power at the mobile reduces the level of interference for all other users. The base station uses a slightly different scheme. It repeats the same bit patterns as many times as needed to get back to the full rate of 9600 bps. The transmit power for that channel is adjusted to reflect this repetition which allows the interference to be minimized. Repeating the bits at lower power is more effective on the forward link than it could be on the reverse link due to the use of a coherent phase reference called the pilot signal.

19 CDMA Frame Formats 9600 bps Frame 14400 bps Frame 4800 bps Frame
192 bits in a ms frame 288 bits in a ms frame 9600 bps Frame 14400 bps Frame 171 12 8 266 12 8 Encoder Tail Bits 1-bit Reserved Mixed Mode Bit Encoder Tail Bits Information Bits CRC Mixed Mode Bit Information Bits CRC 96 bits in a ms frame 144 bits in a ms frame 4800 bps Frame 7200 bps Frame 79 8 8 124 10 8 Encoder Tail Bits Encoder Tail Bits 1-bit Reserved Mixed Mode Bit Information Bits Information Bits CRC Mixed Mode Bit CRC 48 bits in a ms frame 72 bits in a ms frame 2400 bps Frame 3600 bps Frame 39 8 54 8 8 Encoder Tail Bits Encoder Tail Bits 1-bit Reserved Mixed Mode Bit Information Bits Information Bits Mixed Mode Bit CRC 24 bits in a ms frame 36 bits in a ms frame 1200 bps Frame 1800 bps Frame 15 8 20 6 8 Encoder Tail Bits Encoder Tail Bits 1-bit Reserved Mixed Mode Bit Information Bits Information Bits Mixed Mode Bit CRC

20 Mobile Power Bursting Each Frame is Divided into 16 Power Control Groups Each Power Control Group Contains 1536 Chips (represents 12 encoded voice bits) Average Power is Lowered 3 dB for Each Lower Data Rate CDMA Frame = 20 ms Full Rate Half Rate Quarter Rate Eighth Rate

21 Walsh Codes W = 0 1 0 0 0 1 W W W = W = n n 2 2n n n An important feature of the forward link is the use of Walsh codes. These have the characteristic of being orthogonal to each other and to the logical NOT of each other. Two codes are defined to be orthogonal if they have an exact zero cross product when summed over the full period of the codes. Walsh codes are generated by the expansion shown below: The variable, n, in this expansion must always be a power of two. This is seeded with the one by one matrix: The TIA CDMA system uses a 64 by 64 Walsh matrix (each Walsh code is 64 bits long). W = 4

22 Forward Link Traffic Channel Physical Layer
Power Control Puncturing Vocoded Speech data Convolutional Encoder Mbps Interleaver 800 bps Walsh Cover I Short Code 9.6 kbps Long Code Scrambling 19.2 kbps 1/2 rate 1.2288 Mbps P.C. MUX I FIR 3/4 rate 19.2 kbps 19.2 kbps 19.2 kbps 19.2 kbps Short Code Scrambler 14.4 kbps Q FIR 1.2288 Mbps 20 ms blocks 19.2 kbps 800 bps Long Code Walsh Code Generator Voice data at 9600 bps (full rate) is first passed through a convolutional encoder, which doubles the data rate. It is then interleaved, a process that has no effect on the rate, but does introduce time delays in the final reconstruction of the signal. A long code is XOR'ed with the data, which is a voice privacy function and not needed for channelization. CDMA then applies a 64 bit Walsh code which is uniquely assigned to a base to mobile link to form one channel. This sets a physical limit of 64 channels on the forward link. If the coded voice data is a zero, the Walsh sequence is output; if the data is a one, the logical not of the Walsh code is sent. The Walsh coding yields a data rate increase of 64 times. The data is then split into I and Q channels, and spread with short codes. The final signals are passed through a low pass filter, and eventually sent to an I/Q modulator. Q Short Code Mbps

23 Long Code Generation Modulo-2 Addition Long Code Output 3 4 1 2
User Assigned Long Code Mask 42 bits 41 42 5 Long Code Generator The Long Code is generated using a 42 bit linear feedback shift register. This is the master clock and is synchronized in all CDMA radios. A specific mask is applied to generate a unique long code.

24 Forward Link Channel Format
Walsh Code 32 Walsh Code 0 Pilot Channel Sync Channel Walsh Codes 1 to 7 Walsh Codes 8-31, 33-63 Traffic Channels 1 up to 55 Channels All 0's 19.2 kbps kbps I Convert to I/Q & PN Spreading FIR LP Filter & D/A Conversion Q 4.8 kbps I Data Q Data Paging Channels 1 up to 7 Channels The Base Station transmitter signal is the composite of many channels (with a minimum of four). The Pilot channel is un-modulated; it consists of only the final spreading sequence (short sequences). The Pilot Channel is used by all mobiles linked to a cell as a coherent phase reference. The other three channels are the Sync channel, the Paging channel, and the Traffic Channel which use the same data flow, but different data are sent on these channels. The Sync channel transmits time of day information. This allows the mobile and the base to align clocks which form the basis of the codes that are needed by both to make a link. The Paging channel is the digital control channel for the forward link. Its complement is the access channel which is the reverse link control channel. One base station can have multiple paging channels and access channels if needed. The Traffic channel is equivalent to the analog voice channel. This is where the actual conversations take place.

25 Reverse Link Traffic Channel Physical Layer
Interleaver I Short Code Q Short Code 1.2288 Mbps I Q 307.2 kbps t/2 1/2 Chip Delay 28.8 kbps 20 ms blocks Vocoded Speech Data 64-ary Modulator Mbps 1 of 64 Walsh Codes Long Code FIR Walsh Code 1 Walsh Code 2 Walsh Code 0 Walsh Code 62 Walsh Code 63 Walsh Code 61 Convolutional Encoder 9.6 kbps 1/3 rate 1/2 rate 14.4 kbps Long Code Modulator Short Code Scrambler The CDMA reverse link uses a different coding scheme to transmit data. Unlike the forward link, the reverse link cannot support a pilot channel for synchronous demodulation (since each mobile station would need its own pilot channel). Due to this limitation, the reverse link has less capacity than the forward link. To aid reverse link performance, the 9600 bps voice data uses a one-third rate convolutional coded for more powerful error correction. Then six data bits at a time are taken to point at one of the 64 available Walsh codes. The data which is at kbps [28.8 kbps x 64 walsh bits / 6 bits) = kpbs] is then XOR'ed with the long code to reach the full Mbps data rate. This unique long code is the channelization for the reverse link.

26 CDMA Modulation Formats
Base Station Transmitter Mobile Station Transmitter I Q I Q The modulation is Filtered QPSK in the base station, and Filtered Offset QPSK in the mobile station. Note that the I/Q diagram for the base station signal is for only a single channel (such as the pilot channel). In normal operation, many channels are summed together and transmitted on top of each other by the base station. O-QPSK is used in the mobile stations because it avoids the origin and makes the design of the output amplifier easier. For the base station, since many channels are summed together, using O-QPSK would not always avoid the origin. This is due to random nature of adding many signals together. Filtered QPSK Filtered Offset QPSK

27 Ten Minutes in the Life of a CDMA Mobile Phone
System access Continue travel Initiate Soft Handoff Terminate Soft Handoff End call Turn-on System access Travel Idle State Hand-Off Initiate call

28 CDMA Turn On Process Find all receivable pilot signals
Choose strongest one Establish Frequency and PN Time Reference (Base station I.D.) Demodulate sync channel Establish system time Determine paging channel Long Code Mask System Access: When the mobile first turns on, it must find the best base station. This is similar to analog where the phone scans all the control channels and selects the best one. In CDMA, the mobile unit scans for available Pilot signals, which are all on different time offsets. This process is made easier because of the fixed offsets. The timing of any base station is always an exact multiple of 64 system clock cycles ( called chips) offset from any other base station. The mobile selects the strongest pilot tone and establishes a frequency and time reference off this signal. The mobile then demodulates the sync channel which is always on Walsh 32. This channel provides master clock information by sending the state of the 42 bit long code shift register 320 milliseconds in the future. The Sync Channel also contains many other system parameters. The mobile then starts listening to the paging channel, and waits for a Page that is directed to its phone number. The mobile will often register with the base station so that the base station can do location-based paging rather than system wide paging. Once the mobile has read the sync channel and established system time, the mobile uses the parameters from the sync channel to determine the long code mask being used by the cell site it is acquiring.

29 CDMA Service Options Service Options Are:
1- Voice Using 9600 bps IS-96-A Vocoder 2- Rate Set 1 Loopback (9600 bps) 3- Voice Using 9600 bps (EVRC) 4- Asynchronous Data Service (circuit switched) 5- Group 3 Fax 6- Short Message Service (9600 bps) 7- Internet Standard PPP Packet Data 8- CDPD Over PPP Packet Data 9- Rate Set 2 Loopback (14400 bps) 14-Short Message Service (14400 bps) 32,768- Voice Using bps (CDG)

30 CDMA Protocol Stacks EIA/TIA-95 Rev B J-STD-008 TBS- 74 IS -95 Rev A
Combines TSB-74 & J-STD-008 for a Universal Protocol J-STD-008 Not Backwards Compatible, PCS only Protocol TBS- 74 Cellular Protocol that adds Channel Support ARIB T53 Japan CDMA System Cellular Protocol IS -95 Rev A Backwards compatible with IS-95. First Deployed Protocol IS -95 Rev 0 Original System-never actually deployed

31 SYNC Sync Channel Message Contains the Following Data:
Base Station Protocol Revision Min Protocol Revision Supported SID, NID of Cellular System Pilot PN Offset of Base Station Long Code State System Time Leap Seconds From Start of System Time Local Time Offset from System Time Daylight Savings Time Flag Paging Channel Data Rate Channel Number SYNC

32 Paging Channel Messages
J-STD-008 Paging Messages Overhead Messages System Parameters Access Parameters CDMA Channel List Extended System Parameters Extended Neighbor List Other Messages Order Channel Assignment Data Burst More Messages Authentication SSD Update Feature Notification Status Request Service Redirection General Page Global Service Redirection TMSI Assignment

33 CDMA Idle State Handoff
No Call in progress Mobile Listens to new Cell Move Registration Location if entering a new zone The mobile has searchers scanning for alternative Pilot tones at all times. If a Pilot tone is found from another base station that is strong enough for a link, the mobile will request a soft handoff. In this case, no call is in process, so it is an idle state handoff. This is an active process that updates the location of the mobile to the system.

34 CDMA Call Initiation Dial numbers, then press send
Mobile transmits on a special channel called the Access Channel The Access Probe uses Long Code Mask based on: Access & Paging Channel Numbers Base station ID Pilot PN offset The user then decides to make a call. The number is keyed in and the send key is hit. This initiates an Access Probe. The mobile uses the Access Channel and attempts to make contact with the serving base station. As no link is yet established, closed-loop power control is not active. The mobile uses open-loop control to guess an initial level. Multiple tries are allowed with random times between the tries to avoid collisions that can occur on the Access Channel. After each attempt, the mobile listens to the Paging Channel for a response from the base station. The base station responds with an assignment to a traffic channel. This is a Walsh code for the forward link. The traffic channels uses different long code mask than the paging channel. The base station initiates the land link, and a conversation can take place.

35 CDMA Call Completion Base answers Access Probe using the Channel Assignment Message Mobile goes to a Traffic Channel based on the Channel Assignment Message information Base station begins to transmit and receive traffic channel

36 CDMA Soft Handoff Initiation
Mobile finds second Pilot of sufficient power (exceeds T_add Threshold) Mobile sends Pilot strength message to first Base station Base station notifies MTSO MTSO requests New Walsh Assignment from second Base station If available, New Walsh Channel Info is relayed to first Base station During the call, the mobile finds yet another base station with good power. The mobile makes a request from its serving cell to initiate soft handoff with the additional cell. The base station passes this request to the MTSO (Mobile Telephone Switching Office) which contacts the second base station and gets a Walsh assignment.

37 CDMA Soft Handoff Completion
First Base station orders Soft Handoff with New Walsh Assignment MTSO sends Land Link to second Base Station Mobile receives Power from two Base Stations MTSO chooses better quality frame every 20 milliseconds This is sent to the mobile by the first base station. The land link is connected to both base stations. The mobile combines the signals from both base stations by using the two Pilot signals as coherent phase references. At the MTSO, the signals are examined from each base station and the better one is chosen for each 20 ms block.

38 Ending CDMA Soft Handoff
First BS Pilot Power Goes low at Mobile Station (drops below T_drop) Mobile sends Pilot Strength Message First Base station stops transmitting and frees up Channel Traffic channel continues on Base station Two As the signal from the first base station degrades, the mobile will ask that the soft handoff be terminated. At this point the mobile is being power controlled by the second base station (since the first cell probably has a very poor link). The request is passed from the second cell through the MTSO, and the first cell stops transmission and reception of the signal. The mobile is now only on the second cell.

39 CDMA End of Call Mobile or land initiated
Mobile and Base stop transmission Land connection broken Finally, the call ends. This can be initiated either from the mobile or the land side. In either case, transmissions are stopped and the land line connection is broken.

40 CDMA Conclusions New access method
Code based Designed for use in interfering environment Uses multipath to advantage Has high capacity 6 to 20 times analog CDMA Conclusions: CDMA provides an advanced technology for Cellular applications. It provides high quality service to a large number of users. It is a system that has been extensively tested. Its use will continue to expand throughout the world.

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